The present invention relates to a novel process of preparation and intermediate for the preparation of lithium positive electrode active materials for use in lithium secondary batteries.
Lithium positive electrode active materials may be characterised by the formula LixNiyMn2-yO4-δ wherein 0.9≤x≤1.1, 0.4≤y≤0.5 and 0≤δ≤0.1. Such materials may be used for e.g.: portable equipment (U.S. Pat. No. 8,404,381 B2); electric vehicles, energy storage systems, auxiliary power units (APU) and uninterruptible power supplies (UPS). Lithium positive electrode active materials are seen as a prospective successor to current lithium secondary battery cathode materials such as: LiCoO2, and LiMn2O4.
Lithium positive electrode active materials may be prepared from precursors obtained by a co-precipitation process. The precursors and product are spherical due to the co-precipitation process. Electrochimica Acta (2014), pp 290-296 discloses a material prepared from precursors obtained by a co-precipitation process followed by sequential sintering (heat treatment) at 500° C., followed by 800° C. The product obtained is highly crystalline and has a spinel structure after the first heat treatment step (500° C.). A uniform morphology, tap density of 2.03 g cm−3 and uniform secondary particle size of 5.6 μm of the product is observed. Electrochimica Acta (2004) pp 939-948 states that a uniform distribution of spherical particles exhibits a higher tap density than irregular particles due to their greater fluidity and ease of packing. It is postulated that the hierarchical morphology obtained and large secondary particle size of the LiNi0.5Mn1.5O4 increases the tap density.
Electrochimica Acta (2010) pp 832-837 also discloses an increase in the secondary particle size of spinel LiNi0.5Mn1.5O4 material by calcination of the material at increasing temperatures, e.g. greater than 800° C. According to the Electrochimica Acta (2004) pp 939-948 reference, it may be assumed that the increase in particle size increases the tap density.
Lithium positive electrode active materials may also be prepared from precursors obtained by mechanically mixing starting materials to form a homogenous mixture, as disclosed in U.S. Pat. No. 8,404,381 B2 and U.S. Pat. No. 7,754,384 B2. The precursor is heated at 600° C., annealed between 700 and 950° C., and cooled in a medium containing oxygen. It is disclosed that the 600° C. heat treatment step is required in order to ensure that the lithium is well incorporated into the mixed nickel and manganese oxide precursor. It is also disclosed that the annealing step is generally at a temperature greater than 800° C. in order to cause a loss of oxygen while creating the desired spinel morphology. It is further disclosed that subsequent cooling in an oxygen containing medium enables a partial return of oxygen. U.S. Pat. No. 7,754,384 B2 is silent with regard to the tap density of the material. It is also disclosed that 1 to 5 mole percent excess of lithium is used to prepare the precursor.
J. Electrochem. Soc. (1997) 144, pp 205-213; also discloses the preparation of spinel LiNi0.5Mn1.5O4 from a precursor prepared from mechanically mixing starting materials to obtain a homogenous mixture. The precursor is heated three times in air at 750° C. and once at 800° C. It is disclosed that LiNi0.5Mn1.5O4 loses oxygen and disproportionates when heated above 650° C.; however, the LiNi0.5Mn1.5O4 stoichiometry is regained by slow cooling rates in an oxygen containing atmosphere. Particle sizes and tap densities are not disclosed. It is also disclosed that the preparation of spinel phase material by mechanically mixing starting materials to obtain a homogenous mixture is difficult, and a precursor prepared by a sol-gel method was preferred.
It is desirable to increase the tap density of battery materials as an increase in tap density may increase the energy density of the battery. Additionally, it is desirable to produce a spinel phase material corresponding to the formula LiNixMn2-xO4 that requires a smaller excess expensive starting materials, such as materials comprising lithium, that may have fewer process steps (heating steps), and/or is applicable to any precursor regardless of the process of its preparation.
It is an object of the present invention to provide a lithium positive electrode active material intermediate and a process for preparing a spinel lithium positive electrode active material corresponding to the formula LiNixMn2-xO4 that has a high tap density equal to or greater than 1.8 g cm−3. It is additionally desirable that this material has a high capacity equal to or greater than 110 mAhg−1 at a current of 30 mAg−1, and high stability wherein the capacity of the material decreases by no more than 7% over 100 cycles between from 3.5 to 5.0 V at 55° C., and up to 2% over 100 cycles between from 3.5 to 5.0 V at room temperature.